For several years we have been studying regulation of protein synthesis by membrane-bound versus free polyribosomes in liver. We have translated liver mRNA in both homologous (liver) and heterologous (reticulocyte) cell-free systems. Synthesis of albumin and ferritin has been used as a marker for specific secretory versus intracellular proteins. Alterations in synthesis of these proteins and their respective mRNA levels in normal, regenerating, and toxically injured liver cells are currently in progress. The major purpose of these studies is to determine whether levels of specific mRNA in a given ribosomal population or specific protein factors may regulate protein synthesis in the liver. In other studies we have become interested in mechanisms for mRNA release cytoplasmic transport, and polyribosomal attachment in eukaryotic cells. Our current feeling is that mRNA released from the nucleus exists in the form of messenger-ribonucleoprotein particles (mRNPs) and that specific protein factors in these particles may influence mRNA metabolism or function in eukaryotic cells.